The present invention relates to systems and methods for metal plating and etching of substrates. More particularly, the invention relates to metal plating and etching of readily oxidizable substrates or other substrates with thin layers that inhibit plating and etching.
Metal plating of articles or base substrates is a common industrial practice. A metal layer may be coated or plated on the surface of an article, for example, for decoration, reflection of light, protection against corrosion, or increased wearing quality. Articles or base substrates, which are made of metal or non-metallic material, may be plated with suitable coating metals using techniques such as electroplating, electroless plating, metal spraying, hot dip galvanizing, vacuum metallization or other available processes. Plating by electrolysis, or electroplating, is a commonly used technique for metal plating because it permits the control of the thickness of the plating. Cadmium, zinc, silver, gold, tin, copper, nickel, and chromium are commonly used plating/coating metals. In immersion or electroless plating, some metals are directly precipitated, without electricity, from chemical solutions onto the surface of the substrates. The silvering of mirrors is a type of plating in which silver is precipitated chemically on glass. Any of the common metals and some nonmetals, e.g., plastics, with suitably prepared (e.g., etched) surfaces can be used as the article or base substrate material.
However, some metals (e.g., aluminum and refractory metals like tungsten, tantalum and molybdenum), which have desirable physical or structural properties for use as base substrate material, are extremely difficult to plate by simple immersion plating or electroplating techniques. The difficulty in plating these metals may, for example, be related to the propensity of these metals to oxidize in air, as a result of which an interfering metal-oxide or insulating layer forms on any exposed or etched surface of these metals. The interfering metal-oxide or insulating layer hinders reduction of metal ions, which is required to cause metal plating. Therefore, techniques for metal plating readily-oxidizable materials (such as tungsten, tantalum and aluminum) commonly involve a number of expensive and tedious substrate preparation steps, which are designed to avoid or prevent the formation of surface layers which can interfere with the plating processes. For example, a common technique for metal plating onto an aluminum substrate involves first zincating and then gold plating the aluminum substrate prior to plating the aluminum substrate with a metal of choice. For substrates or articles made from refractory metals such as tantalum and tungsten, the substrate preparation steps prior to metal plating often involve cumbersome high temperature processing steps.
The interfering surface oxide layers formed on these readily-oxidizable metals also hinders etching of the surface of these metals, which may be necessary prior to any substrate preparation steps themselves. The surface oxide layer coating prevents the dissolution of the metal under conventional etching conditions. Again, a number of fairly harsh steps are required to prepare the substrate surfaces for etching. See e.g., Modern Electroplating (3rd edition), F. Lowenheim, Ed. John Wiley & Sons Inc. (1974), pp. 591-625. Further discussion of electroless plating of common materials that require multistep processing to achieve metal plating due to presence of interfering surface films may, for example, be found, in Electroless Plating: Fundamentals and Applications, Glenn O. Mallory and Juan B. Hajdu, Eds. American Electroplaters and Surface Finishers Society (1990), pp. 193-204.
Consideration is now being given to improving metal plating systems and methods. Attention is particularly directed to simplifying techniques for metal plating of substrates on which interfering surface films form, for example, during conventional metal plating processes or steps. Further, attention is directed to substrate preparation techniques (i.e., removal of native or preformed surface oxide layers) prior to plating or etching action.